1. Field of the Invention
This invention relates generally to magnetic recording hard disk drives (HDDs), and more particularly to a HDD with improved reading of data in the presence of inter-track interference (ITI).
2. Description of the Related Art
In HDDs, data is written in angularly spaced data sectors in the concentric data tracks of the recording layers on the disks. A data sector preamble precedes each data sector and is written each time data is written in the data sector. The preamble includes a synchronization field (SF) for achieving bit synchronization and a synchronization mark (SM) for indicating the start of the data sector. The writing of the preamble and the data is initiated by the opening of a write gate in the read/write electronics. However, the write gate signal may exhibit jitter so that the preambles in radially adjacent tracks are not precisely aligned circumferentially (i.e., in the along-the-track direction). The result is that there can be track-to-track phase error, which can prevent the read head from achieving bit synchronization or achieving bit synchronization with the data sectors in the wrong data track. An additional problem in achieving bit synchronization arises because of decreased track spacing and inherent track misregistration (TMR) during reading and writing. To meet the demand for HDDs with high data density, the data track spacing or “pitch” has decreased, which means that the concentric tracks are packed closer together. Also, because the heads cannot be precisely positioned radially, there is always some TMR during reading and writing. The result can be track “squeeze”, which means that the preambles for data sectors in one track can be partially written over the preambles in radially adjacent tracks. Thus when the read head attempts to read the preamble from one track it will pick up some signal from the preambles in a radially adjacent track due to track squeeze.
The problems caused by write gate jitter, TMR and track squeeze can result in significant inter-track interference (ITI). If ITI is too great, reliable acquisition of synchronization is not possible or the read head may incorrectly synchronize with data sectors in an adjacent data track.
What is needed is a disk drive with the ability to reliably achieve bit synchronization and thus reliably read data even in the presence of significant ITI.